The optical region of the electromagnetic spectrum is generally understood to extend from the far ultraviolet wavelengths (about 0.36 .mu.m) to the near infrared wavelengths (about 1.10 .mu.m). The selection of optical glasses for designing lens systems that are color-corrected at more than two wavelengths in the optical region has been discussed by a number of investigators, including:
(a) N.v.d.W. Lessing, J. Opt. Soc. Am. 47, 955 (1957) PA1 (b) N.v.d.W. Lessing, J. Opt. Soc. Am. 48, 269 (1958) PA1 (c) N.v.d.W. Lessing, Appl. Opt. 9, 1665 (1970) PA1 (d) R. E. Stephens, J. Opt. Soc. Am. 49, 398 (1959) PA1 (e) R. E. Stephens, J. Opt. Soc. Am. 50, 1016 (1960) PA1 (f) M. Herzberger, Optica Acta 6, 197 (1959) PA1 (g) M. Herzberger and N. R. McClure, Appl. Opt. 2, 533 (1963) PA1 (h) M. Herzberger, Optik 35, 1 (1972) PA1 (i) H. Drucks, Optik 23, 523 (1966) PA1 (j) H. Schulz, Optik 25, 203 (1967) PA1 (k) H. Schulz, Optik 25, 208 (1967) PA1 (l) H. Pulvermacher, Optik 30, 297 (1969) PA1 (m) H. Pulvermacher, Optik 30, 396 (1970) PA1 (n) M. Herzberger and H. Pulvermacher, Optik Acta 17, 349 (1970) PA1 (o) B. L. Nefedov, Sov. J. Opt. Techonol. 40, 46 (1973) PA1 (p) A. B. Agurok, Sov. J. Opt. Technol. 44, 114 (1977) PA1 (q) M. G. Shpyakin, Sov. J. Opt. Technol. 45, 81 (1978) PA1 (r) M. G. Shpyakin, Sov. J. Opt. Technol. 45, 219 (1978) PA1 (s) G. A. Mozharov, Sov. J. Opt. Technol. 44, 146 (1977) PA1 (t) G. A. Mozharov, Sov. J. Opt. Technol. 47, 398 (1980). PA1 (a) Determining the dispersion coefficients for each type of optical material, the dispersion coefficients being coordinates defining a unique "glass point" for each type of optical material in an (n-1)-dimensional "glass space" coordinate system, where n is the number of wavelengths for which color correction is desired; PA1 (b) Calculating the slope of the hyperline connecting the origin of the "glass space" coordinate system with the "glass point" for each type of optical material; PA1 (c) Sorting the various types of optical materials in ascending order of the slopes of the hyperlines connecting the "glass points" with the origin of the coordinate system; and PA1 (d) Selecting a pair of optical materials for which the hyperline slopes are substantially equal.
An optical "multiplet" is an optical system comprising a number (designated generally herein by k) of refracting elements made of a number (designated generally herein by q) of different optical materials, where each refracting element is made entirely of a particular optical material, with the various refracting elements in combination providing color correction at a number (designated generally herein by n) of wavelengths. The refracting elements could be configured as lens elements, prism elements, or anamorphic elements, depending upon the nature of the optical system. The term "color correction" as used herein with respect to dioptric and catadioptric systems means correction of axial chromatic aberration. Thus, to say that a lens system or a Mangin mirror system is color-corrected at n wavelengths means that n wavelengths are brought to a common focus on the optic axis of the system. For a prism system, "color correction" means correction of chromatic dispersion from the total deviation angle of the system.
Various combinations of optical glasses were discovered in the prior art for designing lens systems that bring more than two wavelengths to a common focus. Previous lens systems, which were color-corrected at more than two wavelengths, generally required three or more different types of optical glasses for making three or more lens elements. There was little success in the prior art in identifying suitable pairs of optical glasses for designing lens doublets (i.e., lens systems comprising only two lens elements) capable of bringing more than two wavelengths to a common focus.
U.S. Pat. No. 3,395,962 to Herzberger, et al. described a photographic objective comprising three lens triplets, with each triplet being individually color-corrected at four wavelengths. M. Herzberger, in reference (f) on the above list, suggested that color correction of an optical lens system at four wavelengths is substantially equivalent to color correction for the entire spectrum bounded by the most widely separated of those four wavelengths.
A lens system comprising only two lens elements, i.e., a doublet, is inherently simpler than a lens system comprising a triplet or higher-number multiplet of lens elements. In terms of fabrication cost and compatibility with constraints imposed by the practical considerations involved in designing optical instruments, a lens doublet that is color-corrected at three or more wavelengths would ordinarily be preferable to a higher-number lens multiplet color-corrected at the same number of wavelengths. A computer-aided search to identify pairs of optical glasses suitable for designing lens doublets that are color-corrected at four wavelengths was made without success by R. R. Willey, Jr., as reported in Appl. Opt. 1, 368 (1962). However, Willey did report that certain pairs of glasses can be used for designing lens doublets color-corrected at three wavelengths.
An article by M. Gaj and J. Nowak in Optik 29, 321 (1969) stated that lens triplets color-corrected at three wavelengths could be designed using various pairs of glasses listed in the optical catalog published by Glaswerke Schott of Mainz, West Germany. However, two of the pairs of glasses identified by Gaj and Nowak were said to require an additional fluorite lens element to produce the desired color correction at three wavelengths. Gaj and Nowak did not report any design data necessary for verifying the design and for constructing lens elements made from the optical materials identified in their article.
U.S. Pat. No. 3,674,330 disclosed particular pairs of alkali halide crystals from which lens doublets that are substantially achromatic over a broad spectrum of wavelengths in the far infrared region, i.e., at wavelengths longer than 1.5 .mu.m can be designed. However, there have been no reports in the literature of any crystal pairs that can be used for designing lens systems color-corrected at more than two wavelengths in the optical region.
The 1977 article by A. B. Agurok, which is reference (p) on the above list, reported on an investigation of the glasses listed in the Russian optical glass catalog GOST (All Union State Standard) to identify pairs of optical glasses suitable for designing lens doublets color-corrected at more than three wavelengths. The search indicated that no two glasses in the GOST catalog can be combined to make a lens doublet that is color-corrected at four wavelengths. Agurok also reported that no two glasses in the Schott catalog had been found, which in combination make a lens doublet that is color-corrected at four wavelengths.
The various algorithms that were developed in the prior art for selecting optical glasses for designing color-corrected lens systems made use of conventional glass parameters (i.e., Abbe numbers and relative partial dispersion ratios) to characterize the dispersive properties of the available types of optical glasses. The approaches taken in the prior art, however, never led to an algorithm of general validity for identifying compatible combinations of optical materials for designing lens multiplets that bring more than two wavelengths to a common focus.
Until the present invention, the problem of correcting optical systems for chromatic aberration has been one of the most difficult problems facing optical designers. A theoretically rigorous procedure for selecting compatible pairs of optical glasses for designing lens doublets that are color-corrected at more than two wavelengths has eluded previous researchers. Non-rigorous "cut-and-try" methods have failed to identify compatible pairs of optical glasses for designing lens doublets color-corrected at more than three wavelengths. C. G. Wynne, in an article entitled "Secondary Spectrum Correction with Normal Glasses", Optics Communications 21, 419 (1977), noted: "It has been generally accepted that the correction of secondary spectrum aberrations, in optical imaging systems, necessarily requires the use of glasses having abnormal relative partial dispersions. This is . . . an error, arising from defects in the accepted theory of first order chromatic aberrations."
The inability of the prior art to develop a unified theory for selecting optical materials to use in designing optical systems that are color-corrected at more than two wavelengths can be attributed to the unavailability, until now, of a tractable mathematical model for representing optical materials in terms of their dispersion characteristics.